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A couple of days ago someone (Mickey Rowe?) posted a comment on a new
iridium paper that was published in Science that had implications for the
K-T boundary. The only reference given was the issue date and it turned out
to be incorrect. I tracked down the reference (using Science Magazine's web
site) and the correct reference is...
Anbar, A. D., Wasserburg, G. J., Papanastassiou, D. A. & Anderson, P. S.,
1996. Iridium in natural waters. Science, 273, 1524-1528.
The paper reports on several things: 1.) the amount of Ir in seawater (it's
apparently the rarest stable element therein). 2.) the flux of Ir from
rivers (it's actually quite high in rivers that drain industrialized
areas), the ability of natural processes to concentrate Ir (there are quite
a few ways), and (most importantly from the standpoint of K-T) 4.) the
residence time of Ir in seawater. This residence time turns out to be quite
long: 2,000 - 20,000 years. The authors also comment if a large body
impacted the Earth, and contained enough Ir to overwhelm the terrestrial Ir
signal it could hang around in the environment long enough to account for
the thicker Ir anomalies (e.g., the Gubbio anomaly) which have been
estimated to represent 10,000 to 100,000 years of deposition. Previously...
Rocchia, R., Boclet, D., Bronté, P., Jéhanno, C., Chen, Y., Courtillot, V.,
Mary, C. & Wezel, F., 1990. The Cretaceous-Tertiary boundary at Gubbio
revisited: vertical extent of the Ir anomaly. Earth and Planetary Science
Letters, 99, 206-219.
...had argued that the only way to obtain an Ir anomaly over that kind of
interval is via a terrestrial volcanic source. Anbar et al. (1996) accept
the Rocchia (1990) interpretation, but point out that, due to the long
residence time of Ir in seawater, a single input of Ir could produce the
same effect.
What is interesting for me in this paper is the additional confirmation
that Ir anomalies represent long (ecologically, very long) periods of time.
A lot can happen to the environment in 10,000 to 100,000 years. I think
it's also interesting to note that many previous interpretations of
sedimentary sequences containing Ir anomalies imply that the Ir deposition
must have taken place in hours to days to (at most) months. Anbar et al.
(1996) appear to explicitly accept the Rocchia et al. (1990) 10,000 -
100,000 year estimate for the duration of the Ir anomaly at Gubbio based on
their measurements of modern Ir marine residence times. Also the thickness
of the Gubbio Ir anomaly was only one of the arguments made by Rocchia et
al. in their re-interpretation.
As you might expect, a lot has been written about the geochemistry and
stratigraphy of Ir in general and K-T Ir in particular. If you are
interested in this topic you might also want to read...
Colodner, D. C., Boyle, E. A., Edmond, J. M. & Thomson, J., 1992.
Post-depositional mobility of platinum, iridium and rhenium in marine
sediments. Nature, 358, 402-404.
Crockett, J. H., Officer, C. B., C., W. F. & Johnson, G. D., 1988.
Distribution of noble metals across the Cretaceous/Tertiary boundary at
Gubbio, Italy: Iridium variation as a constraint on the duration and nature
of Cretaceous/Tertiary boundary events. Geology, 16, 77-80.
Elliot, D. H., Askin, R. A., Kyte, F. T. & Zinsmeister, W. J., 1994.
Iridium and dinocysts at the Cretaceous-Tertiary boundary on Seymour
Island, Antarctica: implications for the K-T event. Geology, 22, 347-355.
Hansen, H. J., Rasmussen, K. L., Gwozdz, R. & Kunzendorf, H., 1987.
Iridium-bearing carbon black at the Cretaceous-Tertiary boundary. Bulletin
of the Geological Society of Denmark, 36, 305-314.
Lerbekmo, J. F., Sweet, A. R. & St. Louis, R. M., 1987. The relationship
between the iridium anomaly and palynological floral events at three
Cretaceous-Tertiary boundary localities in western Canada. Geological
Society of America Bulletin, 99, 325-330.
Robin, E., Boclet, D., Bonté, P., Froget, L., Jéhanno, C. & Rocchia, R.,
1991. The stratigraphic distribution of Ni-rich spinels in
Cretaceous-Tertiary boundary rocks at El Kef (Tunisia), Caravaca (Spain)
and Hole 761C (Leg 122). Earth and Planetary Science Letters, 107, 715-721.
(This also contains Ir data for these sections/cores).
Sawlowicz, Z., 1993. Iridium and other platinum-group elements as
geochemical markers in sedimentary environments. Palaeogeography,
Palaeoclimatology, Palaeoecology, 104, 253-270.
Tredoux, M., De Wit, M. J., Hart, R. J., Linsay, N. M., Verhagen, B. &
Sellschop, J. P. F., 1988. Chemostratigraphy across the Cretaceous-Tertiary
boundary and a critical assessment of the iridium anomaly. Journal of
Geology, 97, 585-605.
Wang, K., Attrep, M., Jr. & Orth, C. J., 1993. Global iridium anomaly, mass
extinction, and redox change at the Devonian-Carboniferous boundary.
Geology, 21, 1071-1074.
Norm MacLeod
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Norman MacLeod
Micropalaeontological Research
N.MacLeod@nhm.ac.uk (Internet)
N.MacLeod@uk.ac.nhm (Janet)
Address: Dept. of Palaeontology, The Natural History Museum,
Cromwell Road, London, SW7 5BD
Office Phone: 0171-938-9006
Dept. FAX: 0171-938-9277
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